Because of excellent transparency properties, transparent plates such as reinforced glass plates and methacrylic resin plates are used as light guides of optical shutters, and planar light-source devices such as backlights and lighting appliances, for example.
As light guides for the optical shutters, patent publication 1 describes reinforced glass plates, methacrylic resin plates and the like, and their surfaces are treated to be roughened, for example, to be frosted surfaces. Such optical shutters use light guides with their shutter regions entirely roughened. At the end surface of the light guide, an LED (light emitting diode) light source is placed. The optical shutter enables the back side to be visible from the front side when the LED light source is off, but invisible when the LED light is on, because the light diffused at the roughened surface is emitted from the main surface of the light guide. Namely, the optical shutter is open when the light source is off, but closed when the light source is on.
Also, as a light guide in an edge-lit backlight, patent publication 2 describes a methacrylic resin plate with recesses formed on its surface. The light guide used here has recesses each having a diameter of roughly 260 to 450 μm formed by a laser, and is suitable for a planar light source in a liquid crystal display device.